Recently, as the amount of information to be processed by various types of information processing and audiovisual devices has been further increasing, more and more attention is paid to information storage media, which allow the user to easily access the data stored there, which can store a huge size of data, and which can effectively contribute to reducing the sizes of those devices. Meanwhile, various methods have been developed and proposed to store information on an information storage medium as densely as possible. As such an information storage medium with huge storage capacity (or high density), an information storage medium, from/to which information is read or written using a recorder/player that has an optical head including a laser light source with a wavelength of approximately 400-410 nm and a condenser lens with a numerical aperture (NA) of 0.85, has been proposed. For example, there is an information storage medium that can store data of approximately 25 GB per storage layer, i.e., approximately 50 GB for two storage layers combined. Also, some people are trying to increase the storage capacity by stacking a number of storage layers one upon the other with a transparent layer sandwiched between them (see Patent Document No. 1, for example).
The structure of a multilayer information storage medium as disclosed in Patent Document No. 1 will be described with reference to FIG. 3.
FIG. 3 is a cross-sectional view of the multilayer information storage medium. This storage medium includes a substrate 401, transparent layers 402 to 404, a transparent coating layer 409, an L0 layer 410 arranged between the substrate 401 and the transparent layer 402, an L1 layer 420 arranged between the transparent layers 402 and 403, an L2 layer 430 arranged between the transparent layers 403 and 404, and an L3 layer 440 arranged between the transparent layer 404 and the transparent coating 409. The L0 through L3 layers are information storage layers.
To perform a read or write operation on this multilayer information storage medium, the medium is irradiated with a laser beam that has come through the transparent coating layer 409, and one of the L0 to L3 layers is accessed by shifting the focal point of the laser beam, which has been converged by the objective lens of a read/write head, to the target layer, thereby reading or writing data. Supposing the thicknesses of the transparent layers 402, 403 and 404 are identified by A, B and C, these transparent layers satisfy the inequality C<A<B. This inequality is preferably satisfied to prevent an interlayer crosstalk (or noise) from being produced due to interference between the laser beam (read radiation) that has been focused on, and then reflected from, the target L0 or L1 layer 410 or 420 and another laser beam that has been accidentally focused on, and then reflected from, another information storage layer that is located closer to the transparent coating layer 409 than the target layer is.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-213720        